The present invention generally relates to actuatable diffraction gratings and more particularly, to actuatable diffraction gratings providing reduced polarization dependent loss.
An important characteristic of desired for optical telecommunications components is that they have low polarization-dependent loss (PDL). PDL is defined as the degree to which an optical device attenuates an input signal as a function of polarization, commonly expressed in terms of a logarithm the ratio of the diffraction efficiency of transverse electric (TE) polarized light and transverse magnetic (TM) polarized light. Conventional diffraction gratings (i.e., one-dimensional gratings having a single elongate dimension) are intrinsically polarization-dependent devices. Accordingly, MEMS-based (i.e., made using microelectromechanical system manufacturing techniques) conventional diffraction gratings have suffered from significant PDL.
Modifications to MEMS-based diffraction gratings have been made to reduce PDL. One such modification involves using wider elements, either for the entire grating or by making selected grating elements wider. Such modifications provide reduced PDL at a specific actuation state of the MEMS grating, but do not provide significant reduction of PDL over all actuation depths. Additionally, the use of wider grating elements results in larger devices, and smaller diffraction angles. Both of these drawbacks have led to increased package size for devices employing gratings that have been so modified.
An example of a diffraction grating design having equal diffraction efficiency for TE-polarized light and TM-polarized light (i.e., a PDL of zero) is a bi-grating. A bi-grating is a two-dimensional grating which diffracts light in two orthogonal planes. The grating described in U.S. Pat. No. 6,188,519 B1, to Johnson, issued Feb. 13, 2001 is an example of a MEMS-based bi-grating for use in maskless lithography and high resolution printing. The device described in Johnson requires an actuatable membrane and fixed islands, which are difficult to fabricate. Furthermore, when the device described in Johnson is actuated, the membrane is not flat, resulting in a reduction in diffraction efficiency.
Some aspects of the present invention apply a recognition that in an actuatable, one-dimensional diffraction grating, the PDL contribution caused by two grating elements is a function of the relative displacement of the two grating elements (i.e., the gap size; gap size is defined as separation in the z-direction, as illustrated in FIG. 2a); and a further recognition that PDL as a function of displacement has regimes of positive and negative PDL (see FIG. 1). Each grating element of a diffraction grating has a reflective surface that is either integrated with the grating element (i.e., the grating elements is made of a reflective material) or has a reflective surface otherwise disposed on the grating element. The size of a gap is measured between the tops of the reflective surfaces of the relevant grating elements.
Accordingly, by processing light of a channel (e.g., a signal having a single wavelength of light) with a diffraction grating configured such that one or more regions of the grating correspond to a positive PDL (i.e., the relative displacement of at least two grating elements within the grating corresponds to greater throughput efficiency for TE-polarized light than TM-polarized light) and one or more regions correspond to a negative PDL (i.e., the relative displacement of at least two elements within the grating correspond to greater throughput efficiency for TM-polarized light than TE-polarized light L), the overall PDL of the light processed by the grating can be reduced relative to a conventional diffraction grating in which all regions of the diffraction grating correspond to a PDL of the same sign (i.e., all positive or all negative).
One aspect of the present invention is directed to a diffraction grating optical processor having one or more groups of grating elements, each group including three or more grating elements that function together to process light of a channel in a manner that provides reduced PDL relative to a conventional diffraction grating.
The grating elements processing light of a single channel are referred to herein as a xe2x80x9cpixel.xe2x80x9d It is to be understood that it is the aggregate effect of the positioning of all the grating elements of a pixel (i.e., the relative displacements of all grating elements relative to one another) that determines the amount of PDL present in a signal processed by a pixel. However, it is instructive and convenient to ascribe a PDL to individual pairs of grating elements comprising a pixel (e.g., a reference grating element, typically a non-actuatable grating element, and an operational grating element, typically an actuatable grating element); the sum of the PDLs ascribed to the pairs of grating elements are indicative of the PDL for the entire pixel. When referring to the PDL contribution of a pair of grating elements of a pixel (as determined by the gap therebetween), the arrangement will be said xe2x80x9cto correspond to a PDL.xe2x80x9d
Embodiments of actuatable gratings as taught herein may have pixels including a plurality of adjacent actuatable grating elements. Accordingly, gaps (in the z-direction) may be defined between one or more operational grating elements of a pixel and a non-adjacent reference grating element. The gaps in such devices may correspond to positive and negative PDLs, so as to reduce the overall PDL of a pixel (e.g., see FIG. 8).
The present invention also includes, but is not limited to, embodiments of actuatable diffraction gratings that provide reduced PDL over all actuation depths. The phrase xe2x80x9call actuation depthsxe2x80x9d means including actuation distances equal to at least one quarter of a processed wavelength of light, such that a selected amount of diffraction achieved by a given pixel of the diffraction grating may range from zero diffraction (i.e., substantially all of the light reflecting from the pixel remains in the zeroth order) to complete diffraction (i.e., substantially all of the light reflecting from the pixel is diffracted out of the zeroth order).
A first aspect of the invention is directed to an optical processor characterized by an axis extending in a direction, the optical processor comprising: (a) a pixel to process light having a wavelength xcex, comprising (1) a first grating element having a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the axis, (2) a second grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the axis, and (3) a third grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the axis; and (b) a controller operable to displace at least the reflective surface of the second grating element relative the reflective surface of the first grating element, a displacement of the reflective surface of the second grating element forming a first gap in the direction of the axis relative the reflective surface of the first grating element, the first gap corresponding to a PDL of a sign, and the reflective surface of the third grating element forming a second gap, relative one of the reflective surface of the first grating element and the reflective surface of the second grating element, in the direction of the axis and corresponding to a PDL of the opposite sign.
In some embodiments, in a direction perpendicular to the axis, the second grating element is located intermediate the first grating element and the third grating element. In some embodiments, the first grating element and the third grating element are non-actuatable grating elements. The reflective surface of the first grating element and reflective surface of the third grating element may be separated by a distance, in the direction of the axis, equal to an integer multiple of xcex/2. Optionally, the width of the reflective surface of the second grating element is equal to the sum of the widths of the reflective surface of the first grating element and the reflective surface of the third grating element. The PDL of a sign and the PDL of an opposite sign may have substantially the same magnitudes, whereby their sum is substantially zero. The terms xe2x80x9csubstantially zeroxe2x80x9d is defined herein to mean less than 0.2 dB. The controller may be operable to displace the third grating element relative the first grating element, and in a direction perpendicular the axis, the first grating element is located intermediate the second grating element and the third grating element.
In other embodiments, the controller is operable to maintain the reflective surface of the second grating element and reflective surface of the third grating element in positions separated by a distance, in the direction of the axis, equal to an integer multiple of xcex/2 during processing of the light, and wherein the controller is operable to displace the second grating element and the third grating element relative the first grating element during the processing of the light. In still other embodiments, the controller may be operable to displace the first grating element, and the third grating element. In some embodiments, in a direction perpendicular the axis, the first grating element is located intermediate the second grating element and the third grating element, and wherein the controller is operable to maintain the reflective surface of the second element and reflective surface of the third grating element in positions separated by a distance, along the axis, equal to an integer multiple of xcex/2 during processing of the light, and to displace the reflective surface of the first grating element while the reflective surface of the second grating element and the reflective surface of the third element are maintained in said position. In such embodiments, the width of the reflective surface of the first grating element is equal to the sum of the widths of the reflective surface of the second grating element and the reflective surface of the third grating element. The PDL of a sign and the PDL of an opposite sign may have substantially the same magnitudes, whereby their sum is substantially zero.
In some embodiments, the pixel further comprises a fourth grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface normal to the direction of the axis, and wherein, in a direction normal to the axis, the grating elements are arranged in the following order, the first grating element, the second grating element, the fourth grating element and the third grating element. The first grating element and the fourth grating element may be non-actuatable. The first grating element and the fourth grating element may be coplanar with one another. Optionally, the controller is operable to displace the second grating element and the third grating element, and wherein the PDL of a sign and the PDL of an opposite sign have substantially the same magnitudes, whereby their sum is substantially zero. The controller may be operable to maintain the reflective surface of the first grating element and the reflective surface of the fourth grating element in coplanar positions during processing of the light. The controller may be operable to displace the second grating element and the third grating element relative the first grating elements and the fourth grating element, and wherein the PDL of a sign and the PDL of an opposite sign have substantially the same magnitudes, whereby the sum of the PDL of a sign and the PDL of the opposite sign is substantially zero.
In some embodiments, the reflective surface of the first grating element and the reflective surface of the fourth grating element may be non-actuatable, and separated by an integer multiple of xcex/2 along the axis. In such embodiments, the controller may be operable to displace the reflective surface of the second grating element and the reflective surface of the third grating element during processing of the light, and wherein the PDL of a sign and the PDL of an opposite sign have substantially the same magnitudes during the processing of the light, whereby their sum is substantially zero.
In some embodiments, the pixel further comprises a fourth grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface normal to the direction of the axis, and wherein, in a direction normal to the axis, the grating elements are arranged in the following order, the first grating element, the fourth grating element, the second grating element and the third grating element. Optionally, the second grating element and the third grating element are connected together such that reflective surface of the second grating element and the reflective surface third grating element are separated by a distance equal to an integer multiple of xcex/2 in the direction of the axis. The controller may be operable to displace the second grating element and third grating element along the axis, relative the first grating element and the third grating element, whereby the distance is maintained during displacement. The reflective surface of the first grating element and the reflective surface of the fourth grating element may be connected together such that reflective surface first grating element and the reflective surface of the fourth grating element are separated by a distance equal to an integer multiple of xcex/2 in the direction of the axis.
The displacement in any of the above embodiments may be achieved using one of an electrostatic technique, a magnetic technique, a piezoelectric technique, and a thermal technique.
Another aspect of the invention is directed to an optical processor to process light having a wavelength xcex, characterized by an first axis extending in a direction, the optical processor comprising: a first grating element having a length and a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the axis; a second grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the first axis, the second grating element connected to the first grating element such that in the direction of a second axis, which is perpendicular to both the length and the first axis, there is a first step equal in height to a non-zero integer multiple of xcex/4 between the reflective surface of the first grating element and reflective surface of the second grating element; a third grating element having a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the first axis; and a fourth grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the first axis, the fourth grating element connected to the third grating element such that in the direction of the second axis there is a second step equal in height to a non-zero integer multiple of xcex/4 between the reflective surface of the third grating element and reflective surface of the fourth grating element, the first grating element and the second grating element displaceable relative the third grating element and the fourth grating element, in the direction of the first axis; whereby the first step and the second step are maintained in both actuated state and the unactuated state. The optical processor may further comprise an actuator to effect a displacement of the first grating element and the second grating element relative the third grating element and the fourth grating element, in the direction of the first axis. In an unactuated state, the reflective surface of the first grating element may be coplanar with the reflective surface of the third grating element, and the reflective surface of second grating element is coplanar with the reflective surface of the fourth grating element. The actuator may be one of an electrostatic actuator, a magnetic actuator, a piezoelectric actuator, and a thermal actuator.
Still another aspect of the invention is an optical processor to process light having a wavelength xcex, characterized by an first axis extending in a direction, the optical processor comprising: a first grating element having a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the axis; a second grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the first axis, the second grating element having a structurally fixed separation from the first grating element, along the first axis, equal to a non-zero integer multiple of xcex/4; and a third grating element having a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the first axis, the third grating element displaceable relative the first grating element and second grating element.
The optical processor may further comprise an actuator to effect a displacement of the first grating element and the second grating element relative the third grating element, in the direction of the first axis. Optionally, the first grating element and the second grating element are non-actuatable. In some embodiments, in a direction perpendicular the axis, the third grating element is located intermediate the first grating element and the second grating element. The width of the reflective surface of the third grating element may be equal to the sum of the widths of the reflective surface of the first grating element and the reflective surface of the second grating element. The gap between the reflective surface of the first grating surface and the reflective surface of the third grating element, in the direction of the axis, may correspond to a PDL of a sign, and the gap between the reflective surface of the second grating element and the reflective surface of the third grating element, in the direction of the axis, may correspond to a PDL of the opposite sign, the PDL of a sign and the PDL of the opposite sign have substantially the same magnitudes, whereby their sum is substantially zero.
Yet another aspect of the invention is directed to an optical system comprising (a) an optical source to produce a wavelength xcex; and (b) a fixed diffraction grating characterized by an axis extending in a direction, the diffraction grating comprising a pixel configured to receive the wavelength xcex, comprising (1) a first grating element having a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the axis, (2) a second grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the axis, the reflective surface of the second grating element forming a first gap in the direction of the axis relative the reflective surface of the first grating element, the first gap corresponding to a PDL of a sign and
(3) a third grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the axis, the reflective surface of the third grating element forming a second gap, relative one of the reflective surface of the first grating element and the reflective surface of the second grating element, in the direction of the axis and corresponding to a PDL of the opposite sign.
A fourth aspect of the invention is a method of operating a pixel of an optical processor, characterized by an axis, the pixel having (a) a first grating element having a reflective surface supported above a substrate, at least a portion of the reflective surface normal to a direction of the axis, (b) a second grating element having a reflective surface supported above a substrate, at least a portion of the reflective surface normal to the direction of the axis, and (c) a third grating element having a reflective surface, a portion of the reflective surface normal to the direction of the axis, comprising: positioning the reflective surface of the second grating element to form a first gap relative the reflective surface of the first grating element, the first gap corresponding to a PDL of a sign, the reflective surface of the third grating element forming a second gap relative one of the reflective surface of the first grating and the reflective surface of the second grating, the second gap corresponding to a PDL of the opposite sign. The PDL of a sign and the PDL of an opposite sign may have substantially the same magnitudes, whereby their sum is substantially zero. Optionally, the step of positioning includes increasing the first gap and decreasing the second gap. Alternatively, the step of positioning includes increasing the first gap and increasing the second gap.
In some embodiments, the pixel further comprises a fourth diffractive grating element having a reflective surface, at least a portion of the reflective surface normal to the direction of the axis, the method further comprising positioning the reflective surface of the third grating element such that the second gap corresponds to a PDL having the opposite sign.
The pixel may have a baseline position, and wherein the step of positioning the reflective surface of the second grating element includes moving away from the baseline position and toward the substrate, and the step of positioning reflective surface of the grating element includes moving away form the baseline position and away from the substrate. Alternatively, the pixel has a baseline position, and wherein the step of positioning the reflective surface of the second grating element includes moving away from the baseline position and toward the substrate, and the step of positioning reflective surface of the third grating element includes moving away form the baseline position and toward the substrate.
The step of positioning the reflective surface of the second grating element may increase the first gap and the step of positioning the reflective surface of the third grating element decreases the second gap. Alternatively, the step of positioning the reflective surface of the second grating element may be achieved using one of an electrostatic technique, a magnetic technique, a piezoelectric technique, and a thermal technique.